Cell Cycle I - Interphase

The cell cycle Any living cell derives from another that has divided previously. The cell cycle is a process whereby a cell progresses from one cell division to the next. The cell cycle in eukaryotic cells consists of two parts: interphase and M phase. The interphase, in turn, comprises three phases: G1, S and G2. The M phase, on the other hand, comprises two stages: mitosis and cytokinesis. Here we refer mainly to the interphase. When a cell is in interphase, the chromosomes are in varying degrees of chromatin condensation. This scheme represents a diploid cell, 2n, with two pairs of homologous chromosomes shown in different colors. Long chromosomes are homologous to each other; short chromosomes are homologous to each other. Pink chromosomes come from one parent; green chromosomes come from the other parent. In the G1 phase, the cell begins to transcribe and translate genes. It grows progressively and increases in mass and size. When the cell has reached certain magnitudes in mass and size, it progresses into the S phase or DNA synthesis, which lasts from the beginning of DNA replication until all DNA has replicated completely. Transcription and translation are active in this stage, so that the cell continues growing. At the end of the S phase, the cell has become tetraploid, which in this diagram is expressed as 4n = 8. This means that the cell has eight chromosomes in total. The S phase ends here, and the cell goes to phase G2. In this phase, the cell continues growing and prepares for dividing. At the end of the G2 phase the cell has grown, its genetic material has doubled, and it is ready to start the M phase, which takes this name because it is the stage where mitosis and cytokinesis take place. Mitosis is the separation of the duplicated cellular genome into two identical sets of chromosomes. Cytokinesis is the division of one cell into two daughter cells. At the end of each cycle, each of the two resulting cells begins their own cycle. There are several fully differentiated cells such as neurons, erythrocytes and muscle cells, which do not divide at all, as they have left the cell cycle and remain in a phase called G0, without continuing to step S. The cell cycle is highly regulated, and each of its phases includes at least one control point in which the correct execution of the processes is checked. The start of each phase depends on the correct completion of the previous phases. Who is responsible for this control?: CDK enzymes and cyclins. In humans there exist genes called CDKs, which encode cyclin-dependent kinases, also known as CDK enzymes. Cyclins are proteins that are progressively synthesized during phases G1, S, G2, and that are degraded in the M phase of each cycle. Which are the checkpoints of the cell cycle? In the G1 phase, it is verified that sufficient cell mass and growth exist for withstanding the cell cycle. In the S phase, it has to be verified that the DNA is intact, without any damage. In G2, the verification consists in that each DNA sequence has been replicated just once. In the M phase it is verified that the pairs of kinetochores are formed correctly, in such a way that the chromosomes are equally distributed between the two daughter cells. The cycle continues if the cell passes the checkpoints, otherwise the cell comes to a standstill. Errors in the cell cycle may result in defective cells, whereby they are programmed to die. The process of programmed cell death is called apoptosis. In cancer, which is an alteration of the cell cycle, cells multiply indefinitely, because they have lost the ability to block the cell cycle. The cell cycle is clearly, completely, and didactically presented in: Mendoza, Biologia II, Trillas, Mexico Refer to the webpage http://www.mendoza-sierra.org There you may find several answered questionnaires.